Girl sends my boyfriend pictures


hey guys, so I just need some advice. my boyfriend's friend has recently sent him some pretty slutty pictures over Snapchat and he didn't really seem to mind. she knows about me and everything and still sent it to him. while it wasn't a nude, it was the next closest thing. do I let him know it isn't okay or am I being too controlling? she's known to sleep around, despite if a guy has a girlfriend or not. please help. thank you!!